Electron-heavy Hole Research Articles

Lateral Diffusion Limitations of Ingaas/Gaas for Nanostructure Fabrication

ABSTRACTWe have investigated the technique of implantation enhanced interdiffusion (IEI) for optical nanostructure fabrication in strained InxGal-xAs/GaAs quantum wells. Implantation masks with widths from 40 nm to 40 μm were fabricated on the surface of InxGal-xAs/GaAs (x+0.1, 0.2) 3.5 nm quantum well material which was implanted with 100 kV As+ with doses ranging from 5 × 1012 to 8.5× 1013 ions/cm2. After mask removal and a high temperature anneal, cathodoluminescence (CL) spectroscopy was used to investigate the optical properties of the resulting structures. We have measured electron-heavy hole recombination energy shifts due to quantum well interdiffusion of up to 60 meV for the highest doses used here with broad area implants. However, while quantum well emission under large (40 μm) masks is preserved, smaller masks show an emission blue shift not due to ions penetrating through the mask. A simple model of the width dependence of this shift yields an enhanced lateral diffusion length of approximately 1 μm which is many times larger than the lateral straggle of the implanted As+. We conclude that lateral diffusion effects may impose a limit on nanostructure fabrication in the InxGal-xAs/GaAs system with this technique.

Observation of higher confined exciton states in serpentine superlattices by linear polarized excitation spectroscopy

(Al,Ga)As serpentine superlattice (SSL) quantum-wire arrays with a parabolic-crescent cross section have been grown on vicinal GaAs substrates by molecular beam epitaxy. The quantum wires have dimensions on the order of 5 nm and 1D confined valence band states have been reported previously. Here we report on subsequent SSL samples having photoluminescence (PL) linewidths of 5 meV and displaying well-resolved excitation spectra. The data were obtained at 1.4 K and include linearly-polarized PL and PL excitation spectra (PLE) taken normal to the vicinal (100) surface. These data were fitted with effective-mass calculations that include the SSL geometry, valence band mixing, and imperfect lateral composition modulation. In linear polarized PLE (polarized parallel and perpendicular to the quantum wires), we have been able to resolve a SSL-induced heavy-light hole splitting of the ground state exciton. The observed heavy-light hole splitting is of the order of 5 meV with an electron-heavy hole subband separation of 15-20 meV. We extract a lateral composition modulation between the barrier and the wire to be 15% from the observed heavy-light hole splitting corresponding to a lateral potential-energy difference in the conduction band of about 120 meV

Proton implantation intermixing of GaAs/AlGaAs quantum wells

We have investigated proton implantation enhanced intermixing of GaAs/AlGaAs quantum wells for H+ doses ranging from 5×1013 to 1×1016 ions/cm2. Implantation of 20 keV H+ followed by a high temperature rapid thermal anneal leads to enhanced diffusion of Al into the GaAs quantum well. Shifts of electron–heavy hole recombination energies due to compositional changes were observed using room temperature cathodoluminescence. Diffusion lengths of longer than 2 nm were calculated from energy shifts in a 5 nm well and were found to vary with both implanted dose and anneal time, as expected if the enhanced interdiffusion is caused by implantation introduced defects.

Large, low-voltage absorption changes and absorption bistability in GaAs/AlGaAs/InGaAs asymmetric quantum wells

Three-step asymmetric coupled quantum wells have unique excitonic properties, particularly under bias. We demonstrate these properties through the absorption changes in quantum well optical modulators. The samples consisted of p-i-n diodes with an active region of 20 coupled wells, each coupled well containing a 50 Å GaAs well and a 20 Å In0.2Ga0.8As well separated by a 10 Å Al0.33Ga0.67As barrier. Analysis of the structure shows that field-induced enhancement and suppression of electron and hole envelope wave function overlap can be observed through a corresponding increase or decrease in exciton absorption peaks. Our devices showed suppressed absorption with bias for the electron-heavy hole 1 exciton and enhanced absorption with bias for the electron-heavy hole 2 exciton. Stress-related effects on the electron-light hole 1 exciton are also observed. Absorption change per applied bias is five times lower than at the zero-field exciton wavelength in quantum well devices utilizing the conventional quantum-confined Stark effect (QCSE). At higher bias, the QCSE becomes dominant, producing absorptive bistability. Our devices exhibit lower chirp and lower-voltage operation than single-well devices and the flexibility of design allows for further optimization of absorption changes.

Luminescence from Si1−xGex/Si quantum wells grown by Si molecular-beam epitaxy

Luminescence from strained Si1−xGex/Si quantum wells (QWs) grown on Si substrates by Si molecular-beam epitaxy was studied. Influenece of the crystal quality on the luminescence efficiency was investigated by excitation dependence of luminescence intensity. Electroluminescence (EL) from coupled double QWs for the material system and the associated peak red shift is presented for the first time. Transverse-electric-polarized EL was observed from cleaved edge samples for the first time, showing that the optical transition in QW luminescence is due to electron–heavy hole recombination.

Growth and characterization of A1As/GaInAs multiple quantum wells

We have investigated several A1As/Ga x In 1- x As multiple quantum wells (MQWs) grown on {001} GaAs substrates, consisting of 10 QWs of different thicknesses and In content varying from 1- x=0.08 to 1- x=0.15, separated by ∼ 12 nm thick A1As barriers. Samples have been grown by MBE using different substrate temperatures for the growth of A1As barriers and GaInAs wells. Detailed double-crystal X-ray analysis has been done in all the samples, including dynamical calculations. High quality spectra and good agreement with simulated rocking curves allowed us to determine the structural parameters of each sample, both In content (strain status) and thicknesses of barriers and wells. We have measured the energies of the optical transitions associated to E 0: first and seconf order electron-heavy hole, e-light hole and e-spin orbit transitions. Both type I and type II band alignments are observed and discussed in comparison to results of k · p calculations.

Dependence of polarization, gain, linewidth enhancement factor, and K factor on the sign of the strain of InGaAs/InP strained-layer multiquantum well lasers

The sign of the strain in a multiquantum well (MQW) active layer of an InGaAs/InP laser determines whether lasing occurs from the electron–heavy hole transition or from the electron-light hole transition. Lasing from the electron-light hole transition is reported to provide a much better performance than predicted by theory. It is concluded that this gives the best device performance, providing a higher differential gain, a lower threshold current, a record low linewidth enhancement factor of 1.5, and a K factor of 0.22 ns, potentially allowing a 3 dB modulation bandwidth of 40 GHz.

Electric field-induced optical waveguide intensity modulators using GaAs/AlxGa1−xAs quantum wells

We demonstrate a new waveguide intensity modulator that uses the field-dependent refractive index change associated with quantum wells to control the lateral confinement of light in a slab waveguide. By operating at photon energies ∼40 meV below the zero field electron–heavy hole transition energy, we show, conclusively, that the field-induced refractive index change is primarily due to the quantum-confined Stark effect. Intensity modulators are demonstrated with on/off ratios better than 3:1. Devices based on this electrically controllable lateral confinement will play an important role in integrated optics.

Strained InGaAs/InP quantum well lasers

Quantum well lasers based on strained InxGa1−xAs/InP were grown by atmospheric pressure metalorganic vapor phase epitaxy. Buried-heterostructure lasers with the active layer consisting of three quantum wells, each ∼50 Å thick, placed in a continuously graded waveguide, exhibit threshold currents as low as 15 mA, high quantum efficiency (24%), and power output (∼100 mW), independent of composition. Changing the In concentration from x=0.48 to 0.62 results in the lasing wavelength shift from 1.45 to 1.62 μm. These wavelengths are in excellent agreement with the calculated energies of the electron–heavy hole exciton transition.

Comparison of quantum-confined Stark effect in interdiffused and abrupt GaAs/AlGaAs quantum wells

Photocurrent measurements at room temperature have been carried out on thermally interdiffused GaAs/AlGaAs multiple quantum well structures. Although quite comparable, the Stark shift (quantum-confined Stark effect) in the disordered well is smaller than in the as-grown well. These measurements are consistent with calculations of the electron–heavy hole displacement versus electric field for infinite and square well models for the as-grown well, as well as parabolic and error function well models for the disordered well. We observe that this displacement increases faster for the wells with graded barriers which fit the experimental disordered well. Consequently, the exciton binding energy is smaller, which is consistent with the reduction of the Stark shift revealed by the photocurrent measurements.

Formation of planar superlattice states in new grid-inserted quantum well structures

We have prepared by molecular beam epitaxy a new type of planar superlattice (PSL), in which an array of periodically spaced AlAs bars of one monolayer (ML) in thickness is inserted in the middle of a GaAs quantum well. This grid-inserted quantum well (GI–QW) is prepared on a misoriented GaAs substrate of 2° off from (001) axis by depositing 0.5 ML of AlAs during the growth of a usual QW. Photoluminescence excitation spectra are measured at 20 K and have shown a clear dependence on the polarization of the excitation light, reflecting the in-plane anisotropy of electronic structures. The measured ratio of electron–heavy hole (e–hh) and electron–light hole (e–lh) transition peaks has shown the polarization dependence, which is in good agreement with theory; this demonstrates that 0.5 ML of AlAs atoms deposited on atomic terraces have formed a periodic potential as intended and given rise to the PSL states in this novel structure.

Orientation-dependent phase modulation in InGaAs/GaAs multiquantum well waveguides

The electro-optic effect and phase modulation in In0.2 Ga0.8 As/GaAs multiple quantum wells have been experimentally studied for the first time. The experiments were done with 1.06 and 1.15 μm photoexcitation which are, respectively, 25 and 115 meV below the electron–heavy hole excitonic resonance. Strong quadratic electro-optic effect was observed near the excitonic edge in addition to the linear effect. These are characterized by r63 =−1.85×10−19 m/V and (R33 −R13 )=2.9×10−19 m2 /V2 . In addition, we observe a dispersion in the value of r63 . The relative phase shifts are higher in the strained system at 1.06 μm than in lattice-matched GaAs/AlGaAs.

Excitonic spectrum of an undoped quantum well in electric field

The effect of the valence band coupling on the excitonic spectrum of an undoped GaAsAl xGa 1−xAs quantum well subjected to a normal electric field is examined. The exciton states are split because of the spin-splitting of the hole subbands. The binding energies of the (00h) and (001) excitons are noticeably increased. The transition strength of the “forbidden” (01h) exciton is enhanced both by the electric field and the strong mixing of the hole states. The binding of the 001 exciton is further increased because it exhibits a Fano-like resonance with the electron-heavy hole continuum.

Photoluminescence from “spike doped” hydrogenic donors in Al 0.3Ga 0.7As-GaAs quantum wells

We have performed photoluminescence (PL) measurements on GaAs-Al 0.3Ga 0.7As superlattices where Si donors have been selectively doped at the center of the non-interacting quantum wells. A PL feature is shown to be donor related and is attributed to the radiative recombination of an electron of a neutral donor at the center of a quantum well with a heavy hole. Furthermore, our measurements indicate that the binding energy of the electron bound at a neutral donor is slightly larger than that of a coupled electron heavy hole exciton and, in addition, is significantly larger than that observed for donors in bulk GaAs. These observations are in substantial agreement with recent theoretical calculations.

Effect of substrate annealing and V: III flux ratio on the molecular beam epitaxial growth of AlGaAs-GaAs single quantum wells

Photoluminescence spectra are presented on single quantum well (SQW) structures on thick (1.0 µm) A10.20Ga0.80As buffers grown by molecular beam epitaxy (MBE). Through substrate preparation and careful control of growth conditions, full width half maximum (FWHM) luminescence line-widths of 0.7 meV have been achieved for the n= l confined electron-heavy hole transition. This is the narrowest luminescent linewidth ever reported for any epitaxially grown material. Annealing of substrates with the subsequent removal of the converted surface prior to MBE growth is shown to improve SQW luminescence. Luminescence quality is shown to vary strongly with V: 111 flux ratio during well growth. Improved SQW luminescence is observed for SQW grown using lower V: 111 flux ratios. Growth of a GaAs SQW at 680°Cusing a V:III flux ratio one-half of that required for thick GaAs epilayers while maintaining the same surface reconstruction is demonstrated. The techniques reported are important for the growth of heterojunction interfaces and devices in AlGaAs-GaAs.

Surface state study of ion implanted GaAs (Se) from photoreflectance : W. M. Duncan and A. F. Schreiner. Solid St. Commun.31, 457 (1979)

Photoreflectance and photoluminescence studies were performed to characterize InAs ultrathin layer embedded in Si-delta-doped GaAs/AlGaAs high electron mobility transistors. These structures were grown by Molecular Beam Epitaxy on (1 0 0) oriented GaAs substrates with different silicon-delta-doped layer densities. Interband energy transitions in the InAs ultrathin layer quantum well were observed below the GaAs band gap in the photoreflectance spectra, and assigned to electron–heavy-hole (Ee–hh) and electron–light-hole (Ee–lh) fundamental transitions. These transitions were shifted to lower energy with increasing silicon-δ-doping density. This effect is in good agreement with our theoretical results based on a self-consistent solution of the coupled Schrödinger and Poisson equations and was explained by increased escape of photogenerated carriers and enhanced Quantum Confined Stark Effect in the Si-delta-doped InAs/GaAs QW. In the photoreflectance spectra, not only the channel well interband energy transitions were observed, but also features associated with the GaAs and AlGaAs bulk layers located at about 1.427 and 1.8 eV, respectively. By analyzing the Franz–Keldysh Oscillations observed in the spectral characteristics of Si-δ-doped samples, we have determined the internal electric field introduced by ionized Si-δ-doped centers. We have observed an increase in the electric field in the InAs ultrathin layer with increasing silicon content. The results are explained in terms of doping dependent ionized impurities densities and surface charges.

Warm electron effects in narrow gap Hg1-xCdxTe alloys at ambient temperatures

The predominant scattering mechanism controlling the electrical conductivity of Hg1-xCdxTe alloys is not fully understood in the temperature range 100-400K. Results are reported at 295K for two narrow gap alloys (x=0.20, 0.22) on high field galvanomagnetic effects and warm electron effects for which a novel measurement technique is described. The galvanomagnetic results are consistent with predominant polar optical mode scattering. The warm electron coefficient, beta , was measured to be much larger than can be explained by this mechanism alone or in combination with electron-heavy hole scattering. An effective beta value associated with changes in the Auger recombination and generation rates has been estimated and also shown to be smaller than the measured values.

Energy of free and bound excitons in GaAs in a magnetic field

We have measured in photoluminescence the energy shift of free, weakly and tightly bound excitons in high purity GaAs in magnetic fields up to 12T. In the weak binding limit we find a surprising result: the mass obtained from the magnetic shift is the electron-heavy hole reduced mass in contrast to the results obtained from k · p calculation in zincblende type semiconductors in zero magnetic field. In the tight binding limit the mass deduced from the magnetic shift is the effective electron mass.